The perforant path (PP) innervates dentate granule cells which contain dynorphin and enekephalin. Stimulation of this pathway has been shown to inhibit dynorphin peptide levels and mRNA expression in the hippocampus of young rats. In contrast, basal hippocampal dynorphin content and prodynorphin mRNA are increased in the aged rat brain (PNAS,86,2948,1989). In this study, we examined whether loss of PP input to dynorphin-containing cells in the dentate gyrus of young rats would mimic the effects of aging. Radioimmunoassays (RIA) showed that a bilateral knife cut of the PP at the level of the angular bundle reliably increased dynorphin (20%) and decreased Met-enekephalin (40%) in the ventral hippocampus of young adult Long-Evans rats. Similarly, bilateral entorhinal cortical aspiration also produce a comparable enhancement of dynorphin (20%) in granule mossy fibers and a reduction of enekephlin (40%) in the whole hippocampus. In a second study, the dissected dentate gyrus was assayed for high-affinity glutamate uptake (HAGU) and the CA3 region of hippocampus was asayed for dynorphin immunoreactivity. Bilateral PP lesions significantly decreased HAGU and produced a parallel increase in CA3 dynorphin content. Immunocytochemistry also revealed enhanced dynorphin immunoreactivity in the mossy fibers one month after lesioning. We conclude that removal of the PP elevates dynorphin in the hippocampus and may model the effects of aging on the regulation of this peptide.